x86: Tune Skylake, Cannonlake and Icelake as Haswell

r259399, which added PROCESSOR_SKYLAKE, disabled many x86 optimizations which are enabled by PROCESSOR_HASWELL. As the result, -mtune=skylake generates slower codes on Skylake than before. The same also applies to Cannonlake and Icelak tuning.

This patch changes -mtune={skylake|cannonlake|icelake} to tune like-mtune=haswell for until their tuning is properly adjusted. It also enables -mprefer-vector-width=256 for -mtune=haswell, which has no impact on codegen when AVX512 isn't enabled.